Structural Design of Shallow Foundations

CIVL 451 – Foundation Engineering – Classroom Notes

Design Codes for Reinforced Concrete Foundations

• ACI Committee, American Concrete Institute, & International Organization for Standardization, “Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary,” American Concrete Institute, Detroit, 2011.

• BS 8110. “Structural use of Concrete: Part 1: Code of Practice for Design and Construction,” British Standard Institution, London, 1997.

ACI 318 – 11 Design Philosophy

• Ultimate Limit State Design (USD),

– Various load combinations factored to obtain the most critical load case representing ultimate limit state.

• Load factors mainly account for; accuracy of the estimated design loads, variability of the design loads during the design life of the structure, variability of the results of various analysis methods.

– Material strength reduction factors used to involve a safety factor on the available strength.

– The main principle is such that;

ACI 318 – 11 Design Philosophy

• Serviceability Limit State (SLS),

– Although it is not explicitly mentioned, serviceability check is required to ensure adequate performance of structural members at service load levels.

– This can be achieved by;

• estimation of settlement of foundations for overall performance, and,

• also by estimation of crack width of the designed structure under the impact of the bending moment to satisfy the material performance.

– SLS check also involves checking of the deflection of the foundation system along the footprint of the building structure (discussed in Chapter 2 of the class notes).

Spread (Single, Pad) Foundation Design

• This type of foundation carries load applied by a single column.

• The material used in the construction of spread footings is almost always concrete.

• Could be reinforced or unreinforced.

• Usually only consists bottom reinforcement, sometimes bent up along the sides for better anchorage

Source: http://www.strukts.com/2012/06/types-of-shallow-foundation_94.html

ACI 318 – 11 Background Information

• Spread footings are designed for flexure (bending), shear and bearing.

• Assuming a compressive force is applied via the foundation column, the top side of the foundation becomes under compression and the bottom side will be under tension. Considering a typical section through the foundation;

ACI 318 – 11 Load Factors (9.1 and 9.2)

• Service loads (Design loads) are factored and converted to ultimate loads using the following load combinations;

U: required strength (ultimate load), D: dead load, L: live load, S: snow load, R: rain load, Lr: reduced live load, W: wind load, E: earthquake load, H: soil, soil water or water load.

ACI 318 – 11 Material Strength Factors (9.3)

• The nominal design strength is factored to account for; uncertainty of material strength, variations from designs, quality of construction, accuracy of modelling and analyses, other deviations from design or material behaviour assumptions.

Based on ACI318-86, 9.3

ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)

Bending

• Considering the foundation section as a beam, the assumed stress distribution in foundation section can be approximated as;

max = 0.75 b , min= 0.002 , b = (0.85 1 fc / fy) (600 / (fy + 600) ) ,

where 1 = 0.85 for fc < 27MPa and 1 = 0.85 – 0.05 (fc - 27) / 7 for fc > 27MPa, whilst 1 should not be less than 0.65.

ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)

Shear

• Critical sections;

• Shear stress check governs the thickness design for foundations.

ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)

• Critical sections for moment and shear for foundations;

ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)

• Diagonal tension shear check for foundations;

ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)

• For wide beam shear check use d obtained from DTSC to calculate the applied shear on the critical section as;

Bwbs = B / 2 – (x/2 + d), then

V = Bwbs qu , and V should be < vc for WBS.

Should the above criterion is satisfied, then

it is said that DITSC governs the design for

The foundation thickness, d.

Otherwise, the estimated vc can be

equated to V/bd and the equation solved

For d once again. In that case it is said that

WBS check governs.

ACI 318 – 11 Ultimate State Design (Ultimate Strength Design, USD)

• A typical shallow foundation design involves but are not necessarily limited to the following steps ;

References

• J. N. Cernica, Foundation Design, Wiley.